A fuel cell stack is an electrochemical device that converts chemical energy directly into electrical power. A fuel cell stack comprises a plurality of cells in series. Each cell generates a voltage of about 1 volt, and stacking them allows a higher supply voltage, for example about one hundred volts, to be generated. Among known fuel cell stacks, mention may especially be made of proton exchange membrane (PEM) fuel cell stacks. Each cell of a PEM fuel cell stack comprises a membrane that only allows protons to cross it, between an anode and a cathode, and that blocks the passage of gas. The fuel cell stack may comprise a plurality of flow plates, for example made of graphite, the plates being stacked one on top of another and each plate being associated with a plurality of cells in the stack. The plates may comprise channels and orifices for guiding the reactants and products through the stack. At the anode, hydrogen gas, used as a fuel, is ionized to produce protons that pass through the membrane. The electrons produced by this reaction flow through an electrical circuit external to the cell, forming a current. At the cathode, oxygen is reduced and reacts with the protons to form water.
It is known that there is a risk that the cells of fuel cell stacks will be corroded when they are shut down. This is because, if no electrical load is connected across the terminals of the fuel cell stack during this operation, the presence of residual air on the cathode and the presence of residual hydrogen on the anode may induce inappropriate electrical potentials. Such potentials may oxidize and corrode a carbon support and a platinum catalyst, degrading their performance. This degradation is called Ostwald ripening. This effect becomes more problematic if the fuel cell stack is used in applications that require it to be frequently stopped. Various solutions to this problem have been suggested.
According to a first method, an inert gas is injected into the cells so as to purge the region near the anode and cathode immediately after disconnection of the electrical load. Such inert-gas injection both reduces the degradation in cell performance and prevents, for safety reasons, an inflammable mixture of hydrogen and air, which could appear, from appearing. U.S. Pat. No. 5,013,617 describes such a method, and the connection of an electrical load across the terminals of the fuel cell stack during the gas injection in order to rapidly lower the cathode potential of the cells to a level between 0.3 and 0.7 V.
However, this method has its drawbacks. This method results in a bulky fuel cell stack, which is undesirable in some applications such as automotive applications or mass market electronics. In addition, this method is not suited to applications in which the fuel cell stack is frequently shut down and started up, due to the time required for the purge.
To prevent rapid voltage variation in each cell of a fuel cell stack, document US 2007/0224482 suggests transferring electric charge from each cell to a corresponding energy storage device. This controls the voltage of each cell and increases the lifetime of the fuel cell stack.
Document EP 1 450 429 describes a fuel cell stack comprising a plurality of cells separated by separators. The cells are connected to external resistors, each resistor allowing a current to flow from each fuel cell stack so that corrosion problems related to fuel remaining after the cell has been shut down are solved. A switch is placed in series with the external resistors.
Document AT 505 914 describes a method for shutting down a cell of a fuel cell stack. The fuel cell stack comprises a plurality of cells connected in series. In operation, at least two reactive gases are introduced into the cells, and when operation is stopped, the supply of one of the gases is stopped. The gas remaining in the cells is consumed. The consumption of the residual gas is achieved by connecting an electrical load across the terminals of the cells.
Patent application US 2008/0032163 describes a system for controlling the shutdown and startup operations of a fuel cell stack. The system measures the voltage across the terminals of a number of cells and then determines the cell with the highest voltage and the cell with the lowest voltage. During the shutdown or startup operation, depending on the measured voltages, the system connects an electrical load of higher or lower resistance across the terminals of the fuel cell stack stack. The system sets the impedance of the electrical load so as to keep the voltage across the terminals of the cells between a minimum threshold and a maximum threshold, in order to prevent them from being degraded.
Such a system does not provide an optimal protection of the cells from deterioration.